PROJECT SUMMARY Ongoing B cell differentiation during animal development and at homeostasis in the adult requires the continuous production of cells whose antigen receptors are precisely tuned to the repertoire of antigens that represent self or pathogen. This requires precise control of signaling checkpoints at multiple stages of B cell differentiation. This exploratory proposal investigates a series of chromatin-regulating proteins in one of these checkpoints, ensuring proper levels of the pre-B cell receptor (preBCR) and Ig heavy chain before light-chain rearrangement occurs. Our preliminary data demonstrate a specific role for the MLL1 proto-oncogene and histone methyltransferase in B cell differentiation at the preBCR selection stage. We have identified ineffective preBCR signaling resulting in poor cell survival as defective in a lymphocyte-specific Mll1 knockout animal model. However, the molecular mechanisms resulting in these phenotypes are unclear. This proposal seeks to clarify an important connection between a chromatin-based pathway (MLL1-Sirtuin1) and signaling and survival defects that occur at a very well-characterized stage of B cell differentiation. Specifically, we hypothesize that MLL1 regulates target genes influencing RAS/MAPK/ERK signaling downstream of the preBCR, through a mechanism that is opposed by Sirtuin1 function. The aims proposed identify the nature of the signaling defect by complementing with well-characterized signaling molecules, and use co-deletion of Mll1 and Sirtuin1 to clarify the regulatory hierarchy and relationship between genes controlled by MLL1 and Sirtuin1 in B cells.